In Capnography, monitoring of the concentration of exhaled carbon dioxide is generally performed to assess the physiological status of patients with respiratory problems, including those receiving mechanical ventilation, and to determine the adequacy of ventilation in anaesthetized patients. Two methods are typically employed, Mainstream Capnography and Side-stream Capnography.
In a mainstream Capnography system, a breath measuring device is directly coupled, through an appropriate adapter, to a patient airway tube connecting the patient to a ventilation machine. A sensor is fitted onto the airway tube so that the exhaled breath is detected by the sensor which measures the carbon dioxide concentration in the exhaled breath. The method is generally used with intubated patients as, for non-intubated patients, a mask is usually required which may be uncomfortable for patients in respiratory distress.
In a side-stream Capnography system, a breath sampling device continuously draws samples of exhaled breath from the attached patient airway tube connecting the patient to the ventilation machine. The accuracy in the measurement and analysis of the samples of breath is dependent on a continuous, smooth, laminar flow in the exhaled gases when traveling from the patient to the breath sampling device, such that the effect on the exhaled gas waveform is maintained to a minimum. Hereinafter, the use of the term “minimum effect on the waveform of the exhaled breath sample” or “effect on the exhaled gas waveform is maintained to a minimum” shall be understood to mean that the effect on the waveform will be such that the accuracy of the measurements by the breath sampling device will not be affected. Typical of this method, the breath samples are generally transferred via an airway adapter positioned in the patient airway tube and which includes one or a plurality of breath sampling ports, through a narrow diameter flexible tube, referred to as a breath sampling tube, towards the measuring sensor. A liquid filtering system is used for controlling and handling liquids generally encountered in a medical environment which commonly accumulate in the patient airway tube and enter the breath sampling tube. Some examples of these liquids may be related to patient secretions, condensed-out liquids resulting from high humidity in the ventilation means, and medications and saline solutions provided to a patient during lavage, suction and nebulization procedures.
In a typical side-stream Capnography system, the airway adapter may comprise a tube of approximately 15 mm internal diameter and some 60 mm long, and includes appropriate fittings at either end, serially connected to the patient airway tubing. Midway along the tube may be attached the sampling port, which may be also a plurality of sampling ports, the sampling port comprising a small bore tube, typically 1 mm-2 mm internal diameter, and including one or more inlets. The sampling port is generally positioned perpendicular to the axis of exhaled gas flow in the patient airway tubing, reaching close to the center of the airway adapter tube at one end and exiting the airway adapter tube for connection to one end of the breath sampling tube at the other end. At the other end of the breath sampling tube is connected the breath sampling device, which includes a pump, which may be, for example, a diaphragm pump, which continuously creates a pressure drop at the inlet of the sampling port with respect to the airway adapter. This enables a continuous sample of breath to enter the sampling port and flow through the breath sampling tube to the measurement sensor for analysis (except when blockages occur, as explained below).
The liquid filtering systems are generally designed with three parameters in mind. First, minimize any effect on the waveform of the breath samples to maintain accurate measurements. Second, minimize the possibility of liquids collecting in the breath sampling tube to avoid interference with the monitoring of continuous samples of breath by the breath sampling device. Third, restrict liquids from reaching the sensitive components of the breath sampling device to avoid possible damage to the device. Numerous methods are available which address these parameters; some are based on preventing liquids from entering the breath sampling tube and others are based on removing the liquids once inside the breath sampling tube. The following are a few examples:
a. A specially designed airway adapter fitted on the patient airway tubing to which is connected the breath sampling tube.
b. A Nation (or similar) tube, which may form part of the breath sampling tube, for reducing humidity and consequently reducing the possibility of condensed-out liquids collecting.
c. A reservoir or other liquid collection means in the breath sampling tube adapted to collect liquids flowing into the breath sampling tube, such that the exhaled gas sample may continue to flow undisturbed towards the measurement sensor.
d. A hydrophobic fuse, also referred to as a hydrophobic block or filter, located before the breath sampling device to trap any liquids which have managed to pass the other elements of the filtering system and to prevent them from entering into the device.
e. A measurement sensor, which includes software that is able to detect an inflow of liquid into the breath sampling tube, generally based on sensing an increase in the pressure drop in the breath sampling tube. The sensor may be programmed to react by drawing the liquid into a reservoir or other type of liquid collection element in the filtering system, or to stop the sampling, possibly shutting down the sensor altogether.
Situations may periodically arise wherein relatively large quantities of liquid enter into the breath sampling tube through the sampling port(s) as a consequence of the sampling port(s) being temporarily submerged in liquid. This may occur, for example, when the airway tubing and/or the patient are periodically moved and liquid collecting within the airway tubing splashes from one side to the other, or during lavage when a saline solution is injected into the airway tubing towards the patient's lungs to break down secretions. Another possible occurrence may be when the airway adapter is the lowest point in the airway tubing so that liquids from condensed-out humidity, secretions, medications and other sources collect at the bottom of the airway adapter and splash into the sampling port(s).
When the sampling port is temporarily submerged in liquid, blockage of the sampling port may occur. A blockage refers to a partial or complete obstruction in the flow path of the breath samples due to liquids. This may result in an increase in the negative pressure in the breath sampling tube due to the suctioning action of the pump in the breath sampling device. As the negative pressure continues to increase, a vacuum is created, the vacuum level dependent on the pump strength and the location of the blockage relative to the length of the breath sampling tube. The increased pressure differential between the higher pressure in the airway adapter and the vacuum in the breath sampling tube induces a greater quantity of liquid to flow into the sampling port and into the breath sampling tube. Furthermore, even if the sampling ports have been cleared from any liquid, the liquid accumulated in the breath sampling tube is pulled towards the breath sampling device as the negative pressure further increases due to the pump's suctioning action, the increase a function of the distance the liquid is from the pump.
Breath sampling devices used in Capnography generally include a pressure sensor adapted to measure absolute pressure in the breath sampling tube, necessary for the device to be able to correct for CO2 concentrations as a function of the gas sampled pressure. Typically, commercially available Capnographs are adapted to use the pressure sensor to also monitor blockage conditions such that, when the pressure drop in the breath sampling tube reaches a predetermined threshold, for example, a predetermined value between 60 mbars to 150 mbars, a controller in the Capnograph will interpret this condition to be a blockage condition in the breath sampling tube.
Generally, the controller in the breath sampling device will respond to a blockage by instructing the pump to either increase pumping rate, to continue to pump at the same rate, or to turn off the pump. In each of the first two cases the negative pressure in the breath sampling tube is further increased in the segment between the blockage and the device. This corrective action is taken in an attempt to draw the trapped liquid into one of the liquid collection elements, as previously described, in which case the blockage will be removed. Following a period of pump operation is a time-out period, during which the pump ceases to operate and the controller checks to see if the blockage has been cleared. This is generally done by checking that the pressure sensor is monitoring a second threshold pressure drop where the negative pressure in the breath sampling tube is less than the first threshold. If the negative pressure is less than or equal to the second threshold, then the controller interprets this as the blockage having been removed and the breath sampling device may return to normal operation.
There are numerous drawbacks in the methods previously described for limiting and removing blockages in the breath sampling tube when relatively large quantities of liquid enter into the sampling ports and the breath sampling tube. These are described below:
a. There is a supposition that only a small amount of liquid will enter the sampling port and the breath sampling tube on each occurrence, less than the volume capacity of the liquid collection element, and that the sampling port will be submerged in the liquid for only a relatively short period of time. Nevertheless, this is not always the case, and periodically there may be situations in which relatively large quantities of liquid are sucked into the sampling port and into the breath sampling tube, and then drawn along part or the whole length of the breath sampling tube by the increasing negative pressure resulting from the suctioning pull of the pump in the breath sampling device.b. Design parameters for the liquid collection elements in the liquid filtering system require that the effect on the sampled breath waveform be maintained to a minimum so as to not affect the accuracy of the measurements. Generally, this imposes a series of constraints on the design of the elements, limited by the complex design of the interface between the liquid collection element and the gas flowing portion of the breath sampling tube. The result is that the collection rate of the liquid collection elements is limited to relatively lower rates and, when large quantities of liquid are involved, the liquid often bypasses the liquid collection elements continuing downstream via the breath sampling tube to reach the hydrophobic fuse. There the liquids may block the system, or may even find their way into the device, possibly damaging components in the device.c. Generally the most effective liquid collection elements that cause the least effect on the sampled breath waveform have limited volumes for collection, typically only a few hundred micro liters. This volume contrasts sharply with that of the breath sampling tube which generally has a volume exceeding 2 cc, such that the protection offered by the liquid collection element is limited compared to the capacity of liquid which may be held by the breath sampling tube. As previously mentioned, a large quantity of liquid periodically enters into the breath sampling tube so that, if the liquid collection element is limited in volume, not all the liquid will be retained by the liquid collection elements. This may result in blockage in the breath sampling tube or the liquid continuing downstream via the breath sampling tube to reach the hydrophobic fuse, where it may either block the system or may even find its way into the device, possibly causing damage.d. In those cases where only a filter serves as the liquid collection means (element), due to the very small volume of the filter, it quickly fills up so that all the liquid may not be collected and removed from interfering with the breath sampling. This may result in blockage in the breath sampling tube or the liquid continuing downstream via the breath sampling tube to reach the hydrophobic fuse, where it may either block the system or may even find its way into the device, possibly causing damage.
As discussed above breath sampling systems known in the art are not readily adapted to handle exposure to relatively large amounts of liquids periodically encountered in applications related to side-stream Capnography. These breath sampling systems have a tendency to suffer from blockages, due to the inability of their liquid filtering system to collect the amounts of liquid which periodically flow into the breath sampling tube and which may exceed the volumetric capacity of the liquid collecting elements in the liquid filtering system. Test results show that more than 1 cc of liquid can flow into the breath sampling tube, causing the tube to block. The high frequency of blockages results in the breath sampling system requiring constant supervision and maintenance, and repeated replacement of the breath sampling tube. There is, therefore, a need for a breath sampling system capable of handling exposure to relatively large amounts of liquid which may be periodically encountered in applications related to side-stream Capnography.
The terms “blockage” or “block” may refer to any type of resistance to undisturbed gas flow in any part of a tube, such as a sampling tube. The blockage may be a partial blockage which allows some gas flow in the tube, or a complete blockage which entirely blocks gas flow.